GaN:C epilayers were grown by plasma-assisted molecular beam epitaxy on metalorganic vapor phase epitaxy-GaN/ sapphire templates using CCl4 and CS2 vapors as carbon doping sources. The resulting epilayers were characterized by secondary ion mass spectrometry, photoluminescence, and electrical resistivity among other techniques. Both sources were found to controllably introduce carbon in the concentration range 1 x 10(18)-3 x 10(19) cm(-3) with insignificant contamination by other impurities. The CCl4 source drastically reduced the effective Ga/N flux ratio due to a parasitic reaction, necessitating greatly increased Ga flux to maintain Ga-rich conditions. On the other hand GaN:C was successfully obtained with CS2 with no parasitic reaction. For both sources an anomalous concentration dependence of the carborf incorporation behavior was observed, leading to reduced concentrations of electrically active CN acceptors for higher total doping levels. (C) 2003 Elsevier B.V. All rights reserved.